Phileas
I guess we'll just have to wait and see if/when Easyjet make an announcement. I do agree with you on your second point and several of those sorts of route are at the late planning stage.
LGS6753
As you say, the greater expense of using LCY is offset by higher fares. That surely would give other operators a competitive edge by using SEN, where the lower costs (including reduced flight times) would allow lower fares to be offered. The problem with LCY only having slots available off-peak is that many routes demand peak-time scheduling for the London inbound market, so SEN becomes a viable alternative for early morning and late afternoon arrivals/departures, whose peak times will be the same as for LCY.
There is also, of course, considerable catchment area overlap between LCY & SEN. For a number of smaller European carriers, while LCY may be ideal theoretically, SEN is now starting to be viewed as a very acceptable alternative London arrival point.